1. Field of the Invention
This invention relates to a television receiver set, and more particularly to a device, for use in a television receiver set for receiving television signals of vestigial sideband transmission system, capable of improving the picture quality by suppressing an unwanted signal in the form of a ghost signal contained in a desired reception signal.
2. Description of the Prior Art
Hitherto, it has been a practice to use a delay device and a gain control device for the purpose of suppressing a ghost, wherein a reception signal is branched off, one branched signal is delayed by a period of time corresponding to the delay time of the ghost signal by means of the delay device and adjusted in its amplitude by means of the gain control device, and the thus processed branched signal is subtracted from the other branched signal, thereby suppressing the ghost signal. Accordingly, in this method the suppression is effectively performed only when the ghost signal is delayed by a constant time with respect to the desired reception signal and attenuated by a constant quantity, that is, only when the waveform of the ghost signal is analogous to the waveform of the desired reception signal. Practically, however, the ghost signal is unsimilar to the desired reception signal in many cases, mainly because of the present-day broadcasting system which is the vestigial sideband transmission system.
A television signal a(t) received at an antenna can be expressed by the following equation (1), EQU a(t) = {1 + m.sup.. s(t)} cos .omega..sub.c t + k {1 + m.sup.. s(t-.tau.)} cos .omega..sub.c (t-.tau.) (1)
where, m represents the degree of modulation, s(t) a video signal, .omega..sub.c the angular frequency of a carrier wave, k a ratio of the amplitude of a ghost signal to that of a desired reception signal, and .tau. a delay time of the ghost signal.
In equation (1) the first term and the second term in the right side represent the desired reception signal and the ghost signal, respectively. Assuming that a phase angle .phi. (0 .ltoreq. .phi. &lt; 2.pi.) stands EQU - .omega..sub.c.sup.. .tau. = 2n.pi. + .phi. (n: integer) (2),
The second term in the right side of equation (1) is given as follows: EQU k {1 + m.sup.. s(t - .tau.)} cos (.omega..sub.c t + .phi.) (3).
Accordingly, it will be understood that the carrier wave of the ghost signal received at the antenna is different in phase by an angle .phi. from the carrier wave of the desired reception signal.
Incidentally, under the present-day television broadcasting system of the vestigial sideband transmission system, the low band component of the luminance signal is considered to be transmitted almost under the both sideband transmission system while the high band component of the luminance signal and the color signal are transmitted under the substantial single sideband transmission system. Assuming that a detection stage is the type of a synchronous detection, in case of the both sideband transmission system, the upper sideband component and the lower sideband component appear in a bilaterally symmetric fashion with respect to a symmetric axis of the carrier wave, so that the resultant vector and the carrier wave are always in phase. In consequence, when the synchronous detection is effected with the reference out of phase to the carrier wave by an angle .phi., the signal is multiplied by cos .phi. through the detection, as can be seen from FIG. 1A.
In case of single sideband transmission system, on the other hand, the single sideband component alone appears as shown in FIG. 1B, and therefore the resultant vector varies in both the amplitude and phase. In addition, with respect to one frequency component of the single sideband, the top of the frequency component vector traces a circle with the result that its amplitude is unchanged even when detected with a reference of any angle but its phase is changed in accordance with the angle of the reference. Accordingly, when the synchronous detection is effected with the reference out of phase to the carrier wave by an angle .phi., a signal in the single sideband is detected with the amplitude substantially unchanged, but the time delay of the signal is varied in accordance with the phase difference between the reference and the carrier wave and differs by about several tens nanoseconds in comparison with a detection under the both sideband transmission system. Several tens nanosecond difference in the delay time for the luminance signal does not cause any serious problem on a picture image of television receiver set and is therefore negligible, but when the reference and the carrier wave are out of phase, the amplitude of a detected signal varies in dependence on whether the both sideband transmission system or the single sideband transmission system is employed.
If a desired reception signal contains a ghost signal, since the reference of the synchronous detection and the carrier wave of the desired reception signal are substantially in phase, the reference and the carrier wave of the ghost signal are out of phase by about an angle .phi. in accordance with equations (1) and (3). Accordingly, for the above reason, the low band component of the luminance signal of the ghost signal is multiplied by about cos .phi. while the high band component of the luminance signal and the color signal remain substantially unchanged, and thus the waveform of the detected ghost signal becomes different from that of the desired reception signal. This substantially holds true when the envelope detection is employed.
Therefore, with a ghost suppressing device which comprises a single delay device and a single gain control device, it is impossible, on one hand, to suppress the high band component of the luminance signal of the ghost signal and the color signal when the low band component of the luminance signal can be surpressed and, on the other hand, impossible to suppress the low band component of the luminance signal of the ghost signal when the high band component containing the color signal can be suppressed. With respect to only the luminance signal of the ghost signal, the energy is concentrated almost in the low band component substantially considered to be associated with the both sideband transmission system and the high band component associated with the single sideband transmission system gives little influence upon visual sensitivity. With respect to the luminance signal alone, therefore, it is sufficient to suppress the low band component. For the color signal, however, since the color signal is transmitted along with the high band component of the luminance signal, it is necessary to suppress the high band component in order that the color signal can be suppressed. Further, even when the time delay of the low band component of the luminance signal of the ghost signal from the original signal is equal to that of the color signal of the ghost signal, the phase difference between the low band component and the original signal is not always equal to that between the color signal and the original signal. Consequently, the suppression of either one of the high band component or the low band component fails to assure a complete suppression of the ghost.